There is nothing worse than accepting an applicant who can write an amazing application but doesn't know the first thing about PVE or the class he is playing. Sites like http://www.icy-veins.com/wow/class-guides and http://www.wowhead.com/guides/classes make a good starting point to pick up some helpful pointers about gear, rotations, specs etc. There are also lots of class/spec specific blogs out there with more indepth theory crafting... Make sure you read as much as possible and then take it into practice.

Do some research about the guild you want to join

Are they recruiting my class? If not, chances are still high they will trial good applicants even if they are full. But we will get to that part later.

Do not spam the guild leaders/recruitment officers/members about you wanting to join

This is your first impression so make sure it's a good one. The best way of contacting the people that you need to can be done over PMs on the site.

Its generally a good idea to talk to the Officers in charge, whether it's the Recruitment Officer or Guild Master, before posting an application. You should show that you are interested in being trialed but do this in a calm and relaxed fashion. During this stage you should also ask what the guild environment is like, and any other questions you might have. Try to figure out if you would fit in with the guild before even posting an application. Both you and the guild should benefit from you joining or it might not work out in the long run.

If everything seems on par, you should tell him/her that you will be posting an application. If you are already playing in the same server group this gives you more options. You could tell them to poke you in-game or on IRC if they ever need help with 5 mans, raids, PvP and such, so they can see how you perform.

When/if this happens these will be your most important play sessions! You need to show them that you kick ass and would be a great asset to the guild. Being silent and only speaking when spoken to is a nice way of going about these first sessions. In the meantime, however, you will need to write a top notch application. If the guild you are joining already has a form that you just need to fill out, then make sure you follow that template and make it as detailed as possible, do not leave anything out.

It's better to have too much than too little when it comes to applications.

If you are done with your application in under 10 minutes, chances are it's worthless. Go do something else, come back and try again. The application has to be perfect: it might also help if you spend a few days adding to it and tweaking it around. It has to be something that makes the one reading say; we could really use this player in our roster.

DO NOT SPAM about the status of your application, you can ask about it after a minimum of 1 week without an answer and even then try not to act spammy when you do it, since this can get really annoying really fast.

Make sure you are 100% ready to start raiding straight away

Have boss mods and all guild required raid addons at the ready. Food, Flasks, repair money and everything in terms of PVE knowledge. If there are bosses you have yet to see in the game make sure you read up on those boss guides. You can also try to find good quality boss movies that you can watch to see whats what. But check the strategies your new guild use are the same as the ones you are following!

Finally, when the time comes and you get a trial spot, it's on!

You will now need to perform your arse off: make sure you always play your best, whether you are 1st or 10th on DPS/HPS meters it doesn't really matter as much as being an overall good player. Pushing more HPS and DPS is something you can learn in no time, but being a player who dies on voids, stands in fire or doesn't respond well to advice will get you nowhere.

During the trial period you should try to be as active as possible, even if you aren't used to playing long sessions. It will show the guild leaders that you are an active player and, most importantly, a reliable one. If you've applied as a raider, make sure you raid!

Keep doing what you're doing and you should pass your trial, but don't slack around after you have become a regular member. Always be prepared and make sure you play at the highest level whether it's farm content or progression raids.

I hope this will help at least some of you out there with the drive to aim for the guilds that you one day want to play for.